Age is one obvious reason, sadly books get old. Wear and tear by people is another reason, some people treat books poorly, but hey, to each their own. A lesser known reason is hidden in the materials the book is made from. The acidic damage takes time to develop and become noticeable, but as the books get older the deterioration appears to accelerate.
Where exactly can this acid be found? Actually the acid is in a number of the books components. Acid is found in leather, in the animal skin glues, and in wood pulp paper.
Leather is produced at a high acidic level to keep the leather supple and soft, and so as leather ages the acid-base ratio then shifts to alkaline and the leather becomes unstable. The leather then darkens and becomes brittle and hard, and when it’s placed on an object that flexes, it breaks!
Bookbinders that made books before synthetic glues, did not know that animal skin glues were acidic. Animal skin glues were made in a similar way to leather, to make it flexible and spreadable. Book spines that were made using animal skin glues can get brittle and crack over time!
Paper with a high wood pulp content, also leeches acid into the natural fibers of the pulp, whether it’s cotton, flax or mulberry. Sometimes you can see brown spots on paper, this is called “foxing”, and this is the wood pulp slowly decomposing—leeching acid.
You might have a book with one or all three of these problems, but if any of these materials are acidic the deterioration of your book will most definitely accelerate. Neutralizing, exchanging, or isolating these materials are all part of conservation. Trained conservators know what steps are needed to stabilize the book and help ensure that it will be accessible, in the future, in a safe archival manner.